One non-destructive tomographic measuring technology that has been used in the medical field, etc., is an optical coherence tomography (OCT) that uses temporally low coherence light as a probe (refer to Patent Literature 1). The OCT, as it uses light as a measuring probe, has the advantage of being able to measure the refractive index distribution, spectrometric information, polarization information (double-refractive index distribution), etc., of the measuring target.
Also, its ability to visualize as images the section structure and three-dimensional structure of a living organism at a resolution of approx. 2 to 15 μm in a non-invasive manner with high contrast makes the OCT popular in the fields of ophthalmology, dermatology, dentistry, gastroenterology, cardiology, etc.
The basic OCT 93 is based on Michelson's interferometer and its principles are explained using FIG. 5. The light output from a light source 94 is parallelized by a collimator lens 95 and then split into reference light and object light by a beam splitter 96. The object light is condensed onto a measuring target 98 via an objective lens 97 inside the object arm, where the light is scattered and reflected and travels back to the objective lens 97 and beam splitter 96.
On the other hand, the reference light passes through an objective lens 99 inside the reference atm and then is reflected by a reference mirror 100 and travels back to the beam splitter 96 through the objective lens 99. The reference light, now back at the beam splitter 96, enters the condensing lens 101 together with the object light to be condensed onto an optical detector 102 (photodiode, etc.).
For the OCT light source 94, a source of temporally low coherence light (type of light that almost never interferes with another light output from the same light source at a different point in time) is used. With Michelson's interferometer that uses a temporally low coherence light as its light source, interference signals appear only when the distance from the reference arm and that from the object arm are roughly equal. As a result, measuring the interference signal intensity using the optical detector 102 while changing the differential optical path length (τ) between the reference arm and object arm gives interference signals relative to the differential optical path length (interferogram).
The shape of this interferogram represents the reflectance distribution in the depth direction of the measuring target 98, where the structure of the measuring target 98 in the depth direction can be obtained by one-dimensional axial scan. As described above, the OCT 93 allows for measurement of the structure of the measuring target 98 in the depth direction by means of optical path length scan.
This axial scan may be combined with lateral mechanical scan to obtain two-dimensional tomographic images of the measuring target using the resulting two-dimensional scan. The scanning device with which to perform this lateral scan may be constituted so that the measuring target is moved directly, or it may be constituted so that the objective lens is shifted while the target remains fixed, or it may be constituted so that both the measuring target and objective lens remain fixed while the galvano-mirror positioned near the pupillary surface of the objective lens is angularly rotated, or the like.
Extended forms of the aforementioned basic OCT are the spectral domain OCT (SD-OCT) where a spectrometer is used to obtain spectral signals, and the swept source OCT (SS-OCT) designed to obtain spectral interference signals by scanning the wavelength of the light source. The SD-OCT is classified into the Fourier domain OCT (ED-OCT; refer to Patent Literature 2) and the polarization-sensitive OCT (PS-OCT; refer to Patent Literature 3).
The FD-OCT is characterized in that the wavelength spectra of reflected light from the measuring target are obtained using a spectrometer, after which the obtained spectral intensity distribution is Fourier-transformed to retrieve the signals in the real space (OCT signal space), and with this FD-OCT, the tomographic structure of the measuring target can be measured by scanning it in the x-axis direction, without scanning it in the depth direction.
The SS-OCT obtains three-dimensional optical tomographic images by changing the wavelength of the light source using a high-speed wavelength-scanning laser and then rearranging interference signals and thus processing the signals using the light source scan signals synchronously obtained by the spectral signals. One using a monochrome meter as the means for changing the wavelength of the light source can also be used as the SS-OCT.
The PS-OCT is similar to the Fourier domain OCT in that the wavelength spectra of reflected light from the measuring target are obtained using a spectrometer, but the optical coherence tomographic device for PS-OCT allows for measurement of a finer structure of the specimen and isotropy of its refractive index by capturing polarization information of the specimen (measuring target) (refer to Patent Literature 3).
To explain in greater detail, the PS-OCT is designed to successively modulate the polarized state of the beam that has been linearly polarized at the same time as B-scan, wherein the incident light and reference light are horizontally and linearly polarized, vertically and linearly polarized, linearly polarized at 45°, or circularly polarized, through a ½ wavelength plate, ¼ wavelength plate, etc., respectively, after which the reflected light from the measuring target and reference light are superimposed onto each other and caused to interfere with each other by entering only the horizontally polarized component, for example, of each light into the spectrometer through a ½ wavelength plate, ¼ wavelength plate, etc., so that only the component of the object light in the specified polarized state is retrieved and Fourier-transformed. This PS-OCT does not require scan in the depth direction, either.
The Doppler optical coherence tomography (Doppler OCT) has been known as a type of OCT suitable for ophthalmological examination involving non-invasive measurement of in-vivo blood flows, especially blood flows at the back of the eye (blood flows in the retina), and also suitable as a means for cancer and brain imaging (refer to Patent Literature 4).
The Doppler OCT provides a means for measuring the distribution of blood flows, etc., using the aforementioned FD-OCT, etc., and similarly allows for observation of the dimensional vascular channel structure of the retina by producing cross-sectional retinal blood flow images using the spectral domain OCT.
Another known mode of OCT is one using the Jones matrix (Jones matrix OCT) (refer to Patent Literature 5). In particular, an OCT that uses a fiber to produce Doppler and polarization images is known (Non-patent Literature 1).